We've all seen gyms get crowded in January and thin out by February, as people drop their New Year's resolutions.

So how many stick with it?

Kiip, a mobile-advertising startup which serves nearly 100 fitness-related apps, has revealed data that gives us far better insights into when people start and stop their resolutions.

Analyzing almost 10 million sessions in fitness apps from December to January, Kiip found that there's a definite lift in daily use from December to January.

There's a big lift on January 2—apparently resolutions start then, not on New Year's Day.

Part of the increase comes from new users signing up for the apps. (That might explain why there's a spike in session length for the first couple of days in January, as they experiment with apps for the first time.) But existing app users increased usage 32 percent from December to January.

And usage—time spent, sessions per day, and length of sessions—appeared to be sustained through January.

Of course, people who download fitness apps might be more committed and inclined to keep their resolutions than the general population. Or the apps might help them stick with their resolutions. Since Kiip only studied app users, its study can't address that.